A disk-shaped magnetic recording medium generally has a reference track provided in its innermost circumference and data tracks formed outside the reference track toward the outer circumference of the disk. Data are written in or read from the data tracks by a magnetic head which moves perpendicularly to the circumference direction of the magnetic recording medium. Further, there is a disk-shaped magnetic recording medium on which an optical groove is formed adjoining to each data track. In this case, the magnetic disk unit moves the magnetic head and sets the magnetic head on a track on the basis of the optical groove to thereby read or write data from/in the data track by the magnetic head. The optical groove as a reference for a set-on-track and the data track are usually located apart from each other by several tens of tracks.
To make it possible to exchange such disk-shaped magnetic recording medium among plural magnetic disk units, positioning of the magnetic head should be performed. When the disk-shaped magnetic recording medium has a high track density, accuracy is required for the positioning of the magnetic head.
A conventional method of positioning a magnetic head in a magnetic disk unit as described above is disclosed in Japanese Published Patent Application No. Hei. 2-187969, which discloses that every time a disk-shaped magnetic recording medium is set in the magnetic disk unit, the magnetic disk unit moves the magnetic head to a reference track provided on each disk-shaped magnetic recording medium as an initial operation, and sets an absolute track-zero position from the reference track, thereby performing accurate positioning of the magnetic head.
However, to move the magnetic head to the reference track by the conventional magnetic head positioning method, an operation of performing coarse adjustment which roughly moves the magnetic head to the reference track at the innermost circumference of the recording medium and then performing fine adjustment which slightly moves the magnetic head in the vicinity of the reference track is required. Further, since the conventional magnetic disk unit has an error in a write position of the reference track on the recording medium, an error caused by the accuracy of the coarse adjustment, and an error in a position of the magnetic head with respect to an optical servo due to temperature expansion, a positional range of reference track detection that is performed at the fine adjustment of the magnetic head must be set widely from several tracks to several tens of tracks, which results in a long period of time to position the magnetic head.
The present invention is made to solve the above-mentioned problems. The object of the present invention is therefore to provide a magnetic disk unit which can perform positioning of a magnetic head in a short time.